With a growing demand for communication, optical networks utilizing a wavelength division multiplexing (WDM) technology have become widespread. The WDM technology is a technology in which multiple optical signals each of which has a different wavelength are multiplexed and transmitted. With the WDM technology, for example, multiplexing is performed on optical signals of a transmission rate of 40 Gbps×40 waves, so that a multi-wavelength optical signal of 1.6 Tbps is able to be transmitted.
Examples of a way for error correction in a WDM transmission system include forward error correction (FEC). The FEC exhibits an error correction capability that is in accordance with the redundancy and the code modulation scheme.
As regards the FEC, for example, Japanese Laid-open Patent Publication No. 2003-229815 discloses a WDM transmission system in which FEC data is assigned to wavelength light that is different from that to which main signal data is assigned, and the FEC data is transmitted as an independent signal. With the WDM transmission system, FEC data is not appended to the end of the main signal data. For this reason, it is pointed out that the transmission speed is reduced and the cost of the system is also reduced. Moreover, the WDM transmission system makes it possible to draw a clear distinction between the main signal and the FEC signal. This facilitates the error correction process.
In the WDM transmission system, however, while the transmission capacities of wavelength light are fixed to the same, the amount of FEC data is determined in accordance with the code modulation scheme of FEC and the amount of the main signal data, independently of the transmission capacities of wavelength light. The transmission capacity of the wavelength light assigned to the FEC data is sometimes left over. Accordingly, the above WDM transmission system has a problem in that the transmission efficiency is low.